ATest: Sync auto_test_config for multilib modules

auto_test_config will only be set once for the same
$(my_prefix)$(LOCAL_MODULE_CLASS)_$(LOCAL_MODULE)
ATest should also treat the multilib variant modules as testable.

Fix auto_generated config may have chance to be set as 2nd_arch name

BUG: 111397357
BUG: 110820867
BUG: 80298359

Test: . build/envseup.sh
      lunch aosp_x86_64-eng
      make -j out/target/product/generic_x86_64/module-info.json
      (check module-info.json for hello_world_test)
      make -j hello_world_test
      cat out/target/product/generic_x86_64/testcases/hello_world_test/hello_world_test.config

Change-Id: I69895b6373476c0053f2c6e657770d606ccfbcd1